• • • Eg. blood types A, B, AB and OEach allele results in a different variation of that trait

    A gene codes for a specific polypeptide that can affect a specific trait or characteristic in an organism

    • Eg. blood type

    Alternative forms of a gene?can exist, these various specific forms are called?alleles

    • Note that although alleles are different forms of the same gene, they all still occupy the same locus on the chromosome
    • New alleles occur through?mutations

    Multiple alleles can exist for a gene that determines a specific trait

• The chromosomes of eukaryotic cells occur in?homologous pairs?(there are two copies of each chromosome, one copy inherited from each parent) which means that cells have?two copies of every gene
  • As a result, a cell possesses?two alleles of every gene?within its nucleus
  • When the two alleles at a?locus?are the?same/identical?they are described as?homozygous
  • When the two alleles at a locus are?different?they are described as?heterozygous

Chromosomes showing genes, loci and alleles

• Alleles differ from each other by?one or only a few bases
• Even a very small change in base sequence can bring about a large effect in gene function, with a large knock-on effect on the?phenotype
• Even though different alleles of a gene have slightly?different base sequences,?they still occupy the?same locus?on the chromosome
• Since the Human Genome Project,?sophisticated techniques?can analyse different alleles
• These techniques are becoming?faster,?more accurate?and?more accessible?to individuals
  • Comparable sequences can be analysed down to individual bases to determine?evolutionary relationships
  • The more differences in base sequence, the further apart two species are in evolutionary terms

  The exact positions where bases differ between alleles are called SNPs?or?snips?(Single Nucleotide Polymorphisms)

  • An allele?can have several snips?but still only?differ by a few bases?from its other allele

  Exam Tip